At the moment, if you attempt to edit the Default Value of the "Ranking" custom field that GreenHopper uses (from within Administration>Custom Fields>Rank>Configure), you end up with a stack trace in the browser window (see screenshot).

As the Ranking field is used internally by GreenHopper and is managed automatically it shouldn't be edited anyway, but there ought to be a warning of some sort to this effect, rather than the user attempting to edit the field then getting a stack trace.